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The pulmonary responses of the guinea-pig animal model to inhalation of purified cotton bract extract which was free of endotoxin and chemotactic peptide were examined. These responses were compared to those induced by inhalation of a crude cotton dust extract which contained both endotoxin and chemotactic peptide. An aqueous extract of cotton bract was prepared which contained low endotoxin and nondetectable levels of n-formylmethionyl-leucyl-phenylalanine. Guinea-pigs were exposed for 3 hours to aerosols nebulized from a solution containing 30 milligrams of cotton bract extract per milliliter of sterile water. No changes in pulmonary parameters were noted at the lower two doses of purified cotton bract extract. At the highest dose level, however, increases were observed in breathing rate, macrophage superoxide release as well as bronchoalveolar lavage yield of total cells, red blood cells, lymphocytes, and granulocytes. The authors conclude that, although these pulmonary responses were qualitatively similar to those observed in guinea-pigs exposed to crude cotton dust extract, purified cotton bract extract exhibited somewhat lesser potency.